The broad purpose of the proposed research is to understand the factors controlling 5-HT release and how these change after prolonged administration of ADs. Clinical depression is a devastating disorder. Treatments are available but recovery is usually slow. The effectiveness of drugs that selectively block SSRls suggests that an increase in 5-HT neurotransmission is an important factor in recovery. It is puzzling however, that recovery usually takes weeks although reuptake is rapidly blocked. This delay has been ascribed to a negative feedback mechanism whereby activation of autoreceptors suppresses 5-HT release. Our studies indicate that blocking reuptake causes an immediate, moderate increase in extracellular 5-HT in forebrain, with a further enhancement after prolonged treatment. This application builds on this work, and will again involve microdialysis measurements of 5-HT in the raphe and forebrain sites of freely behaving rats. We will test the hypothesis that the effect of blocking reuptake is smaller in forebrain sites selectively innervated by the dorsal raphe nucleus (DRN) as compared to the median raphe nucleus (MRN). Related to this, our preliminary results suggest the hypothesis that cell body autoreceptors have a stronger inhibitory influence on 5-HT released from DRN than MRN neurons. Conversely, we propose that nerve terminal autoreceptors are more important in regulating release from MRN neurons. Our results do not support the prominent hypothesis that prolonged SSRI treatment results in autoreceptor desensitization. We will test the possibility that enhanced extracellular 5-HT is instead correlated with a decrease in the apparent affinity of autoreceptors. This is based on our preliminary observations that, at high doses, an autoreceptor agonist still strongly inhibited 5-HT release, while the effect of a low dose was greatly attenuated after administration of an SSRI for two weeks. Finally, we will examine the possibility of plasticity in excitatory amino acid (EAA) and gamma aminobutyric acid (GABA)ergic connections with 5-HT neurons. Specifically, we propose that SSRI treatment might lead to either a persistent decrease in the strength of GABAergic, or increase in EM synaptic influences, and that this could offset the inhibitory effect of autoreceptor stimulation leading to enhanced extracellular 5-HT. We suggest that this, rather than altered function of autoreceptors, is the key factor in gradual increases in 5-HT.